The present invention relates generally to testing of special materials, and more particularly to method and apparatus for static fatigue testing of beta" alumina electrolyte material in simulated sodium-sulfur battery cell conditions.
In the operation of a sodium-sulfur cell, both the sodium and sulfur are liquid at the operating temperature of the cell. Sodium ions from the liquid sodium anode are transported through a ceramic electrolyte to the liquid sulfur cathode and surrender charge in a reversible reaction with sulfur which produces sodium polysulfides. The current generated thereby is conducted through the sulfur to a metallic container which is connected to a terminal. Conventional sodium-sulfur cell structures include a solid ceramic electrolyte material which separates the sodium from the sulfur and conducts sodium ions therethrough for reaction with the sulfur. Electrolyte materials typically include beta alumina, beta" alumina, Nasicon (acronym for sodium superionic conductor containing sodium, oxygen, zirconium, silicon and phosphorus) or haloborate glasses. Beta" alumina is a preferred electrolyte material.
In the application of sodium-sulfur batteries for remote uses such as powering systems aboard orbiting satellites, the battery cell structure must have a reasonably long functional lifetime to failure. In conventional structures, failure of the cell occurs predominantly through fracture of the electrolyte. Accordingly, knowledge of structural properties and fracture mechanisms within electrolyte materials in general and within beta" alumina in particular are of critical interest in extending useful lifetimes of sodium-sulfur cells. Previous work has emphasized increase of fracture toughness of beta" alumina.
It is known that beta" alumina can fracture at subcritical stresses as a result of static fatigue. In a particular specimen of beta" alumina material, if crack growth rate increases rapidly as a function of stress intensity factor, increasing fracture toughness in the material will have no significant effect on extending the functional lifetime of electrolyte fabricated from the material. The parameters of interest most logically are, therefore, the threshold stress intensity factor (minimum stress level at which cracks can propagate), the crack growth rate as a function of stress intensity factor, and flaw size within the material from which a crack can propagate. In material having a relatively lesser steep function of crack growth rate versus stress intensity factor, fracture toughness in the material has greater affect on the life of the electrolyte.
It is therefore desirable to provide method and apparatus to mechanically test electrolyte material and more particularly beta" alumina in an environment closely simulating that of an operating sodium-sulfur battery cell incorporating the electrolyte to determine failure parameters and mechanisms of cell materials. No apparatus or method presently exists in the prior art for providing equivalent test procedures or data for electrolyte materials.
The invention meets the deficiency in the prior art just stated by providing method and apparatus for static fatigue testing of electrolyte materials, such as beta" alunina, under conditions simulating that of an operational sodium-sulfur battery. A sample of electrolyte is placed within a controlled inert atmosphere dry box in contact with liquid sodium within a controllable heater; a loading arm extending through the dry box supports a weight table external of the dry box and a loading head in contact with the sample. Transducer means connected to the loading arm measure displacement on the sample as weight is applied to the weight table to the point at which stress fracture of the sample occurs.
It is therefore a principal object of the invention to provide method and apparatus for testing ceramic electrolyte material for use in sodium-sulfur batteries.
It is another object of the invention to provide method and apparatus for testing ceramic electrolyte material under simulated conditions of an operational sodium-sulfur battery.
It is yet another object of the invention to provide method and apparatus for quality control testing of ceramic electrolyte material for a sodium-sulfur battery.
It is yet another object of the invention to provide method and apparatus for measuring physical properties of candidate ceramic electrolyte materials.
It is yet another object of the invention to provide method and apparatus for determining failure mechanisms in sodium-sulfur cells.
These and other objects of the invention will become apparent as the detailed description of representative embodiments proceeds.